Natural gas is the cleanest burning of fossil fuels both with respect to emission of acid gases such as sulfur dioxide and carbon dioxide. For example, compared to coal, burning of natural gas results in the emission of only 60-70% of the CO.sub.2 emissions of a coal burning system.
For the past several years, the perceived abundance of natural gas, advances in gas turbine technology, and many other factors have resulted in significant increases in the use of natural gas for power generation. However, considerable quantities of sub-quality natural gas exist in the United States, and this must be upgraded prior to use. Carbon dioxide is an impurity that creates operational, economic, and environmental problems. It is a diluent without any fuel value, and is an environmental concern as it is one of the greenhouse gases. It is an acid gas and can cause corrosion problems in the presence of water, creating carbonic acid that is quite corrosive to some alloys.
Several CO.sub.2 separation and capture technologies have potential for purification of natural gas. These include amine scrubbing, molecular sieves, cryogenic removal, and membrane separation. Molecular sieves, such as zeolites and activated carbon, are used in pressure swing adsorption (PSA) or temperature swing adsorption systems which separate gas mixtures by selective adsorption of one or more of the gases at high pressure and/or low temperature thus producing a pure product stream. The captured gas is then desorbed by lowering the pressure, or increasing the temperature, of the adsorbent system (thus the system "swings" from a high to low pressure or a low to high temperature).
Carbon fiber sieves are generally known for use in the separation of gases. One such use is described in U.S. Pat. No. 4,734,394 to Kosaka et al., wherein the activated carbon fiber sieve is used to separate nitrogen from air.
U.S. Pat. No. 4,685,940 to Soffer et al. describes a separation device in which carbon membranes have a very narrow range of pore sizes. The membranes are formed by pyrolizing a sheet of regenerated cellulose.
U.S. Pat. No. 5,081,097 to Sharma et al. describes a copper modified carbon molecular sieve for selective oxygen removal.
U.S. Pat. No. 5,411,577 to Moreau et al. describes a method of separating gases using a particulate composite material with a carbon matrix.
U.S. Pat. No. 4,810,266 to Zinnen et al. describes a method of separating carbon dioxide using a carbon molecular sieve. A gas to be treated is contacted with the sieve at room temperature and atmospheric pressure, with adsorbing gas being released by heating to a moderate temperature. The sieve is contacted with a dihydric alcohol amine compound, to thereby impart an amine functionality to the material.
U.S. Pat. No. 4,560,393 to Way describes a method of enriching nitrogen using a pressure swing adsorption system. Electrically negatively charged oxygen is attracted to a positively charged molecular sieve coke material.
U.S. Pat. Nos. 4,094,652 and 4,038,050 to Lowther describe an electrodesorption system for regenerating a dielectric adsorbent bed. A high voltage electric field is applied to zeolite particles that form the molecular sieve bed. Lowther shows that sieve resistivity increases (conductivity decreases) as moisture content decreases. The conductivity of Lowther's bed material is dependent upon its absorbed water content. Lowther's method has been shown useful only for removing water from a gas stream. However, Lowther does not suggest that his method can be used with any other bed material.
U.S. Pat. No. 3,768,232 to Farber describes a solvent recovery method and system including an absorbent bed, a vacuum distillation means, a means for reducing the atmospheric pressure on the bed, a means for heating the bed, a means for condensing the distilled solvent vapor, and a means for collecting the distilled solvent. Farber also describes a plurality of heating elements, such as electrical resistance heating rods containing fins, that are disposed within the porous bed of adsorbent. Farber describes only thermally heating the bed to a temperature at or above the boiling temperature thus distilling solvent from the bed. The present invention describes desorption of gases from their bed using surface physidesorption and ohmic heating, whereas Farber teaches distilling liquids from his bed.
U.S. Pat. No. 5,308,457 to Dalla Betta describes a volatile organic control device comprising an adsorber adapted to adsorb volatile organics from a gas stream passing therethrough and an oxidation catalyst adapted to oxidize the volatile organics desorbed from the adsorber. The present invention uses an electrical current that passes directly through the filter medium, and that both electrically and thermally desorbs sorbed gases. Dalla Betta describes passing an electrical current through a first electrically conductive support for heating, and further describes oxidizing the hydrocarbon by contacting it with a second support having an oxidation catalyst. The present invention desorbs gases from the bed and Dalla Betta oxidizes the hydrocarbons by contact with a catalyst.
U.S. Pat. No. 4,737,164 to Sarkkinen describes a process for recovering volatile impurities from gas which comprises passing the gas through a layer of fibrous activated carbon acting as an adsorbent for impurities while applying an electric voltage across the fibrous layer to improve its adsorption capacity and thereafter desorbing impurities from the layer of fibrous activated carbon. The present invention uses electrical current to desorb pollutants from the filter medium and Sarkkinen uses the electrical current to improve the adsorption capacity of his fibrous activated carbon.
U.S. Pat. No. 5,446,005 to Endo describes an optically isotropic pitch based activated carbon fiber. The present invention describes a carbon fiber composite material consisting essentially of a multiplicity of porous carbon fibers bonded with a carbonizable organic binder.
U.S. Pat. No. 5,091,164 to Takabatake describes a porous carbon-carbon composite formed via a pressure molding process having a porosity of 25%-65%, which is well below the porosity range of 82%-86% described in the present invention.
Pressure swing and temperature swing adsorption are mature technologies and are economically favorable in some gas-gas separation, especially in smaller scale systems. However, a need exists for a more energy efficient desorption technique.
In spite of the above examples of existing separation technologies, a continuing need exists for techniques that are cost effective and relatively simple to operate.